We drew apart, both a little steadier, and smiled nervously. I saw Dougal draw Jamie's dirk from its sheath and wondered why. Still looking at me, Jamie held out his right hand, palm up. I gasped as the point of the dirk scored deeply across his wrist, leaving a dark line of welling blood. There was not time to jerk away before my own hand was seized and I felt the burning slice of the blade. Swiftly, Dougal pressed my wrist to Jamie's and bound the two together with a strip of white linen.
I must have swayed a bit, because Jamie gripped my elbow with his free left hand.
"Bear up, lass," he urged softly. "It's not long now. Say the words after me." It was a short bit of Gaelic, two or three sentences. The words meant nothing to me, but I obediently repeated them after Jamie, stumbling on the slippery vowels. The linen was untied, the wounds blotted clean, and we were married.

(...)

"A blood vow? What do the words mean?"
Jamie took my right hand and gently tucked in the last end of the makeshift bandage.
"It rhymes, more or less, when ye say it in English. It says:

'Ye are Blood of my Blood, and Bone of my Bone.
I give ye my Body, that we Two might be One.
I give ye my Spirit, 'til our Life shall be Done.'

When asked for the origins of this wonderful blood vow, during the Surrey Gathering, in October 1998, Diana Gabaldon told us she had created it. Thus, even though Jamie and Claire say the vow in Gaelic, there's not an official Gaelic version of this blood vow.

You are blood of my veins, you are bone of my bone.
Yours is my body, that we may be one.
Yours is my soul until our worlds end.

Pronunciation

Is too fool o mo chuishlinn, is too cnev de mo chnev.("ch" is pronounced as in “La Cheim”)
Is lahtsa mo vohaig, choom goom bee shinn nar n-aon.("ao" is pronounced as in German "ue" as in Mueller)
Is lahtsa m’anam goose an creeochnich ar sao-al.(Again, the "ch" and "ao" sounds as above.)

For other Celtic prayers and poems with English translations follow this link to the online edition of Carmina Gadelica, by Alexander Carmichael. This link was found by Lady DebbieF, and in her own words "it is one of the world's finest collections of poems, prayers and folklore of the Highlands and Islands in existence. It was written and published in the late 19th century in both English and Gaelic and you may have caught in Companion that Diana Gabaldon uses it as a resource. "